Testosterone masculinizes the development of the substance P-immunoreactive innervation of the rat limbic forebrain

Brown, Sandra A. (1991) Testosterone masculinizes the development of the substance P-immunoreactive innervation of the rat limbic forebrain. Masters thesis, Memorial University of Newfoundland.

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Abstract

Marked sex differences in the substance P-immunoreactive (SPir) innervation of the most posterior divisions of both the medial nucleus of the amygdala (MEPD) and medial bed nucleus of the stria terminalis (BSTMP) have recently been described in adult rats. The present study has examined the effects of early postnatal testosterone exposure on the development of the SPir innervation of these regions and on certain cell groups within the medial amygdala and the BSTMP. Four groups of animals (n=4/group) were compared. Males were either gonadectomized (GM) or sham-operated (SM) on day 1, while females were injected subcutaneously with either vehicle (SF) or 500 ug testosterone propionate (TF) on days 2 and 5. All animals were killed before puberty at day 27. Alternate sections were either stained with cresyl violet or for SP using the peroxidase-anti-peroxidase method. The areas of dense SPir fiber staining and of relevant cell groups (cresyl staining) were measured using a microcomputer-based image analysis system. -- Marked sex differences were seen in the medial amygdale and medial posterior bed nucleus before puberty. The areas of the discrete, darkly-staining fields of SPir fibers in MEPD (MEPD-SP) and in the most medial part of BSTMP (BSTMPM-SP) were larger in sham males than in sham females, as they are in adults. The cell groups MEPD, BSTMP, and the lateral division of BSTMP, BSTMPL, were also significantly larger in sham males than in sham females. The early postnatal treatments completely reversed these sex differences for features of the BSTMP, i.e. measures from groups SM and TF were similar and larger than measures from groups SF and GM which were also similar. Similar results were seen for the sexually dimorphic features of the amygdala, MEPD and MEPD-SP. The areas of these features were not significantly different in groups SF and GM and were smaller than in groups SM and TF. However, these features were larger in group SM than in group TF. Thus injecting newborn females with testosterone did not completely masculinize MEPD and MEPD-SP. Unexpectedly, it was also found that certain features of the medial amygdala and medial bed nucleus were larger in the left than in the right hemisphere. -- The sex differences described here are consistent with the results of recent studies which describe a sexually dimorphic system of heavily interconnected cell groups which includes the vomeronasal olfactory pathway, the most posterior parts of the medial amygdala and medial bed nucleus, and the medial preoptic nucleus. These regions have been implicated in the regulation of a number of sexually differentiated brain functions in the rat. The present data are important because they suggest that SPir neurons form an important part of this sexually dimorphic circuitry, and demonstrate that these sex differences are present before puberty.

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